1. Field of the Invention
The present invention relates to a technique for acquiring image data of a subject and a technique for converting image data.
2. Description of the Background Art
The color vision of a human being has a nature called color constancy that even if a color of a light source for illuminating an object changes with time, a human being can perceive the color as almost the same color if the object is unchanged. From a calculation logic viewpoint, it is considered that color constancy is realized by estimating spectral reflectance of an object as a physical amount peculiar to the object on the basis of light reflected from the object.
Consequently, hitherto, a technique has been proposed, which estimates spectral reflectance of an object (to be expressed more generally, data corresponding to image data obtained by eliminating the influence of an illumination environment (hereinafter, referred to as “object color component data”)) from image data (hereinafter, also properly referred to as just “image”) obtained by a digital camera or the like (see, for example, Japanese Patent Application Laid-Open Nos. 2001-78202 and 2003-169339).
Concretely, the technique disclosed in Japanese Patent Application Laid-Open No. 2001-78202, by using differential image data between image data obtained by capturing an image with electronic flash light and image data obtained by capturing an image without electronic flash light and a spectral distribution of electronic flash light, object color component data can be obtained. From image data based on light from a subject through a predetermined filter, image data based on light from the subject without using the predetermined filter, and spectral transmittance, object color component data can be obtained.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2003-169339, image-capturing is performed with electronic flash light by setting the aperture value to the maximum and setting the shutter speed to the highest, image data of a subject illuminated with electronic flash light is regarded as image data acquired by using only the electronic flash light as illumination light, and object color component data can be obtained from one piece of the image data and the spectral distribution of the electronic flash light.
The conventional techniques described above have the following problems.
In the method of obtaining the object color component data by using the electronic flash light, if a subject is not positioned in a distance where electronic flash light sufficiently reaches, the object color component data of the subject cannot be obtained. In other words, if the spectral distribution of light for illuminating the subject is not known in advance, the object color component data of the subject cannot be obtained. Each time the subject is changed, electronic flash light has to be emitted and a large amount of energy is needed.
On the other hand, in the method of obtaining the object color component data by using a filter, even if the spectral distribution of light illuminating the subject is not known, object color component data of the subject can be obtained so that the energy consumption amount can be reduced.
In the method of obtaining the object color component data by using the filter, however, each time the subject is changed, two image capturing operations consisted of an image capturing operation using a predetermined filter and an image capturing operation without using a predetermined filter have to be performed. Therefore, problems occur such that (i) image-capturing time for obtaining object color component data is long and (ii) a mechanism for driving the filter and the like is additionally needed and the configuration is complicated.